This invention relates to self-cleaning, rotary heat exchangers. It pertains particularly to self-cleaning heat exchangers of the class relying for heat exchange function upon the inclusion of a plurality of Perkins tubes ("heat pipes"). It is described herein with particular reference to heat exchangers employed in conjunction with laundry dryers, although no limitation thereby is intended since it is applicable with equal facility to such appliances as grain dryers, and the various processing units to be found in the textile, food, and fiberboard manufacturing industries.
In the foregoing and other industrial and chemical processes, large quantities of thermal energy in the form of heated gas (usually air) are required to drive off moisture and/or chemical solvents from the materials processed. As a result, the air becomes contaminated not only with moisture or solvents, but also with abrasive particulates emanating from the processed materials and products.
The contaminated air must be discharged from the processing apparatus. It contains valuable residual thermal energy and possibly valuable solvents. It also frequently contains lint, dust, fibers, or other environmentally objectionable materials. In order to recover either the residual thermal energy or the vaporized solvents, the discharged contaminated air must be cooled. Usually, it also must be processed to remove the environmentally objectionable materials.
The application of conventional heat exchangers to the solution of this problem is attended by the difficulty that in the conventional heat exchanger, in order to improve heat transfer and to achieve compactness, metal heat exchanger components are employed in which the metal surfaces are closely spaced, thereby forming small airflow channels. As a result, particulates which are larger than the channel spacings are trapped at the entrances to the heat exchange surfaces and held there by the pressure developed by the flowing airstream. Gradually, these particulates accumulate to form a mat which impedes airflow and, if allowed to accumulate, eventually force the systems to be shut down for cleaning.
Contaminated exhaust air containing solvents also causes problems in small airflow channels. As the airstream is cooled, the solvents condense to form a solvent mist. The mist particles coalesce and adhere to the metal surface by virtue of surface tension. If the solvent is a plasticizer, as is commonly the case in the manufacture of plastic products, the condensed plasticizer gradually polymerizes and forms a solid within the small airflow channels. When this occurs, it is virtually impossible to remove the plasticizer without destroying the metal surfaces.
Contaminated air containing both particulates and solvents is an especially severe environment for heat exchangers. In this case, solid particles which enter the small airflow channels are trapped by the condensed solvent. The particles gradually form a cake which blocks the channels and renders the heat exchanger ineffective.
Particular problems are presented by the operation of the widely used commercial tumble-type laundry dryers through which high velocity heated air is passed. The high velocity hot air detaches lint from the fabrics and carries it out the exhaust of the dryer. The lint consists of fibers and fiber dust. Conventional heat exchange equipment employed to recover the thermal energy exhausted out of such a dryer has proved unsuccessful for two principal reasons:
First, the lint fibers quickly block the small passages.
Cyclone separators have been applied to the solution of this problem; however, they are not efficient in removing the lint. Lint filters also have been employed; however, they too are inefficient and require periodic maintenance.
Second, the cyclone separators and lint filters do not remove the fiber dust.
When this dust reaches the heat exchanger, it settles on the heat exchange surfaces where it is trapped by the laminar boundary layer of the gas flow present in the exchanger. The dust is further held on the heat exchanger surfaces by moisture condensing thereon. Unless the dust is removed by periodic washing, the efficiency of the heat exchanger gradually is reduced. If the cleaning is delayed too long, the dust eventually will form a cake and cleaning by conventional means is very difficult.
Other methods have been proposed to maintain air-to-air heat exchangers operable in contaminated environments.
U.S. Pat. No. 4,025,362 discloses the use of high pressure jets employed periodically to clean the small airflow channels without removing the heat exchanger from operation.
U.S. Pat. No. 4,125,147 discloses the use of perforated endless belts to trap particulates before they enter the heat exchanger.
U.S. Pat. No. 4,068,709 and 4,095,349 disclose easily disassemblable heat exchangers which can be cleaned more easily in the disassembled configuration.
U.S. Pat. No. 4,326,344 discloses a heat exchange system in which lint is removed from contaminated air by means of a cyclone separator. Even with the cyclone the heat exchanger must be vacuum cleaned daily and washed with detergent every two weeks.
It is the general purpose of the present invention to provide a useful rotary heat exchanger which is self-cleaning during operation in many applications.
It is another object to provide a rotary heat exchanger which is adaptable for efficient use in applications involving the processing of hot exhaust gases containing not only particulate contaminants, but also condensible contaminants such as solvents and plasticizers.
It is a further object of the present invention to provide a heat exchanger which recovers efficiently for further use the heat energy content of contaminated hot gases as well as the solvent content thereof.
A further object of the present invention is the provision of a heat exchanger which embodies within a single piece of equipment of simple construction provision for self cleaning and heat transfer, thereby avoiding the necessity for removing equipment from service for periodic cleaning.
Still a further object of the present invention is the provision of equipment which recovers efficiently thermal energy from hot airstreams containing, singly or in combination, dust, lint, fibers, oils, moisture, resins, plasticizers, fats, and other particulates and solvents commonly found in industrial and commercial processes.
The presently described self-cleaning, rotary heat exchanger relies for its heat exchange function upon the presence of an annular array of Perkins tubes.
U.S. Pat. No. 76, 463 describes the construction and mode of operation of the Perkins tube the original purpose of which was to heat a bakery oven without contaminating the baked goods with the combustion gases present in the firebox of the oven.
This was accomplished by partly filling an iron tube with water. Air was removed by boiling the water and letting steam displace the air. After removal of the air, the tubes were hermetically sealed by welding. The tubes then were placed in an inclined position with one end (the evaporation end) in the firebox and the other end (the condensation end) in the breadbaking chamber. Steam generated in the hot evaporation end passed into the relatively cool condensation end where it condensed. The condensed steam (water) thereupon gravitated downwardly into the evaporation end of the tube for repetition of the cycle. Alternatives for gravitational return of the heat exchange liquid include use of an axial wick, (the use of which converts the Perkins tube to a heat pipe), vibration, or centrifugal force.
Rotary heat exchangers involving Perkins tubes as the heat exchange component are known to the art, for example in British Pat. No. 1,600,404, published Oct. 14, 1981; in Japanese Pat. No. 80/01510 (July 24, 1980); and in Japanese Pat. No. 0019691 (Feb. 4, 1983). However, the prior art does not disclose Perkins tube type rotary heat exchangers which are self-cleaning and applicable to the separation of various particulates from a processed gas.